1. Field of the Invention
The present invention relates generally to an optical waveguide module having waveguides and Au electrodes formed on the waveguides, such as an optical modulator module and a polarization scrambler module, and more particularly to an optical waveguide module having good high-frequency characteristics.
2. Description of the Related Art
In recent optical fiber communication systems, a modulation rate has been increased with an increase in transmission rate. In direct intensity modulation of a laser diode, a waveform distortion is caused by wavelength chirping or chromatic chirping. To avoid this problem, an optical modulator module used as an external modulator has increasingly been expected. Further, high-speed optical transmission systems have been put to practical use. In long-haul, large-capacity, high-speed optical transmission systems, a degradation in bit error rate is caused by polarization-dependent gain or loss due to polarization hole burning or the like. As means of solving this problem, a polarization scrambler for varying a polarized condition of transmitted light at high speeds to depolarize the light is effective.
A Mach-Zehnder optical modulator (LN modulator) using a dielectric crystal substrate such as a lithium niobate (LiNbO.sub.3) substrate has been developed as a practical external modulator. Carrier light having a fixed intensity from a light source is supplied to the LN modulator to obtain an optical signal intensity-modulated by a switching operation using interference of light. An LN modulator chip is configured by forming a pair of optical waveguides on the surface of a dielectric substrate of Z-cut lithium niobate crystal, forming a buffer layer of SiO.sub.2 on the pair of optical waveguides, and forming a signal electrode (traveling wave electrode) and a ground electrode respectively corresponding to the pair of optical waveguides on the buffer layer. The optical waveguides are formed by thermal diffusion of Ti into the substrate to increase a refractive index, thereby being connected together in the vicinity of opposite end portions of the substrate.
Signal light entered the optical waveguides from its one connected end is branched to propagate in the pair of optical waveguides. When a drive voltage is applied to the signal electrode formed on one of the optical waveguides, a phase difference is produced between the branched signal lights by an electro-optic effect. In the LN modulator, these signal lights are combined again to be output as an optical signal. By applying a drive voltage to the signal electrode so that the phase difference between the signal lights propagating in the pair of optical waveguides becomes 0 or .pi., for example, a on/off pulse signal can be obtained.
To realize an increased modulation rate in the LN modulator, it is essential to obtain good high-frequency characteristics (attenuation characteristics of microwaves or microwave characteristics of electric reflection). A conventional LN modulator module has a problem such that a modulator chip is not completely grounded to cause a dip of microwave characteristics at a high frequency, resulting in a difficulty of realization of a sufficient wideband performance. This problem arises also in other optical waveguide modules such as a polarization scrambler module and an optical phase modulator module.